Generally, a temperature control of a fixing device of an image processing apparatus is performed based on two temperature threshold vales, i.e., an upper limit temperature and a lower limit temperature between which good fixing performance can be obtained. In the general temperature control, a heat generating element is controlled to generate heat until a temperature of the fixing device reaches the upper limit temperature, and if the temperature reaches the upper limit temperature, the heat generating element is controlled to stop the heat generation. After the heat generating element stops the heat generation, the temperature starts to fall after overshooting for a while. Even if the temperature falls below the upper limit temperature, the heat generating element is kept OFF, and if the temperature reaches the lower limit temperature, the heat generating element is controlled to generate heat. After the heat generating element is turned on, the temperature starts to rise after undershooting for a while. Even if the temperature becomes higher than the lower limit temperature, the heat generating element is kept ON, and if the temperature reaches the upper limit temperature, the heat generating element is controlled to stop heat generation. By repeating such processing, the temperature of the fixing device is controlled. There is a case in which the upper limit temperature and the lower limit temperature are the same, but in that case as well, the control is performed similarly.
However, with the above temperature control, there is a case that the temperature of the fixing device fluctuates in a wide range and the accuracy of temperature control becomes low. In a case where the upper limit temperature and the lower limit temperature are the same, due to the overshooting and undershooting, there is also a case that the temperature of the fixing device fluctuates in a wide range and the accuracy of temperature control becomes low similarly. In order to reduce the range of the fluctuation of the temperature, a method for controlling the heat generating element at a duty ratio of plural values between 0% and 100% rather than controlling the heat generating element at a duty ratio of 100%, is known.
However, if the heat generating element is controlled according to the duty ratio, there is a case that the amplitude of the higher harmonic waves becomes larger.